Hi
Opamps usually have output stage (Class A or Class AB) in order to have low output impedance. Even if an opamp doesn’t have an output stage, since it has a high gain and it is always used in a feedback loop, the output impedance of the opamp is low.
But this is not the case for a transconductor (Gm) cell, as it is used often in an open-loop structure.
Another difference is that the gm of an opamp is not necessarily very linear, while the gm of a transconductor should be very linear.
These are two major differences.
OpAmp